Main

PWM

Auxilary

PWM

Transformer

Current

Trailing Edge

Modulation

Leading Edge

Modulation

Peak Current

Threshold

Peak Current

Threshold

Main

PWM

Auxilary

PWM

Transformer

Current

CRMIX

HS

Phase or CLK

Leading Edge

Modulation

CV

RAMP

RAMP

55k

COMP

PWM

40k

100k

0.7V

CLK

Isync x 3

CRMIX

CV

BUFFER

FB

0.75V

TRK/SS

ERROR

AMP

CRAMP

LM25115A

www.ti.com

SNVS501B – FEBRUARY 2007 – REVISED APRIL 2013

Figure 19. Synchronization and Leading Edge Modulation

Figure 20. Leading versus Trailing Edge Modulation

Valley Current Mode Control

The LM25115A controller uniquely utilizes the elements and benefits of valley current mode control in

conjunction with leading edge modulation to correct changes in output voltage due to line and load transients.

Contrary to peak current mode control, valley current mode control turns on the high-side MOSFET when the

inductor valley current reaches a programmable threshold. This programmable threshold (CRMIX) is the sum of

the output of voltage error amplifier and the RAMP signal generated at the RAMP pin. Valley current mode

control experiences sub-harmonic oscillation when the duty ratio, D, is less than or equal to 50%. Therefore,

adequate slope compensation is needed for the proper operation across the full range of the duty ratio. The

RAMP signal is proportional to the input voltage and it provides the required slope compensation for the valley

current mode scheme. The desired RAMP pin capacitance can be calculated from the following equation:

CRAMP = (0.05 x L) /(RSYNC x RSENSE)

where

•

L is the power inductor

•

RSYNC is the SYNC pin resistor

•

RSENSE is the current sense resistor

The current sense amplifier shown in Figure 21 monitors the inductor current as it flows through a sense resistor

connected between CS and VOUT. The voltage gain of the sense amplifier is nominally equal to 16. The current

sense output signal is shifted by 1.27V to produce the internal CV reference signal. The CV signal is applied to

the negative input of the PWM comparator and compared to CRMIX as illustrated in Figure 21. Therefore when

CRMIX exceeds the PWM threshold (CV), the PWM comparator turns on the high-side MOSFET. Insure that the

Vbias voltage is at least 3V above the regulated output voltage (VOUT) to provide enough headroom for the

current sense amplifier.

Copyright © 2007–2013, Texas Instruments Incorporated

Submit Documentation Feedback

15

Product Folder Links: LM25115A